This proposal comprises two subproposals that use similar methods. 1. Mammalian dipeptidyl peptidase IV[unreadable] (DPP IV) is a highly glycosylated serine protease, that releases N-terminal dipeptides from oligopeptides. It[unreadable] is both an integral membrane protein (CD26) and a plasma protein, the physiological role of which is[unreadable] unknown (Hosono et at., 1999). Among its diverse physiological roles, DPP IV removes X-Pro, His-Ala or[unreadable] tyr-Ala dipeptides from the N-termini of hormones such as neuropeptide Y and substance P. It was[unreadable] discovered in snake venoms by Jorge da Silva and Aird (2000). DPP IVs role in venom may be to prevent[unreadable] a hypertensive response on the part of the envenomated prey by inactivating vasoconstrictive peptidyl[unreadable] hormones (Aird, 2002). It may also contribute to the persistent hypotension seen in human envenomation[unreadable] and to inflammation. The present study should shed light on the role of human soluble DPP IV.[unreadable] 2. Snake venom enzymes that cleave phosphate esters were first discovered when venom[unreadable] phosphodiesterase (PDE) was reported by Gulland and Jackson (1938). Despite this long history, PDE's[unreadable] role in envenomation remained enigmatic until Aird (2002) proposed that its function might be to release[unreadable] endogenous purines, which would immobilize the prey via hypotension and suppression of[unreadable] neurotransmitter release. Snake venom PDE has attracted great interest because of its utility in nucleic[unreadable] acid studies (>2,400 Medline citations). Despite its near reagent status, its structure is unknown. Most[unreadable] forms of PDE are membrane-bound or cytosolic, whereas venom PDE is extremely soluble, so as to[unreadable] function in the extracellular fluid of prey organisms. It has broad substrate specificity [Razzell and Khorana,[unreadable] 1959; Laskowski, 1980], making it well suited to the rapid liberation of adenosine nucleotides from various[unreadable] oligonucleotide precursors, a central theme in envenomation (Aird, 2002). It also has pyrophosphatase[unreadable] activity [Laskowski, 1980], releasing nucleotides and pyrophosphate from nucleoside triphosphates.[unreadable] Nucleotides are rapidly degraded to nucleosides by 5'-nuc!eotidase, present in both venom and prey[unreadable] tissues. Venom PDE was the first enzyme reported to have an active site threonine forming a covalent[unreadable] (phosphorylated) intermediate [Burgers et al., 1979; Gulp and Butler, 1986]. These functional[unreadable] characteristics suggest that it is likely to be structurally unique.